1. Field of the Invention
The present invention concerns methods for radiolabeling proteins and peptides with fluorine-18 (F-18). More particularly, these proteins and peptides are radiolabeled with F-18 by reacting a thiol group contained therein with an F-18-bound labeling reagent which also has a group that is reactive with thiols. The resulting F-18-labeled proteins and peptides are useful in imaging targeted tissue by clinical positron emission tomography.
2. Description of the Related Art
Positron emission tomography (PET) is a high resolution, non-invasive, imaging technique for the visualization of human disease. In PET, 511 keV gamma photons produced during positron annihilation decay are detected. In the clinical setting, fluorine-18 (F-18) is one of the most widely used positron-emitting nuclides. F-18 has a half-life (t.sub.1/2) of 110 minutes, and emits .beta.+ particles at an energy of 635 keV. It is 97% abundant.
The short half-life of F-18 has limited or precluded its use with longer-lived specific targeting vectors such as antibodies, antibody fragments, recombinant antibody constructs and longer-lived receptor-targeted peptides. In addition, complicated chemistry has been required to link the inorganic fluoride species to such organic targeting vectors. In typical synthesis methods, an intermediate is radiofluorinated, and the F-18-labeled intermediate is purified for coupling to protein amino groups. See, e.g., Lang et al., Appl. Radiat. Isol., 45 (12): 1155-63 (1994); Vaidyanathan et al., Bioconj. Chem., 5: 352-56 (1994).
These methods are tedious to perform and require the efforts of specialized professional chemists. They are not amenable to kit formulations for use in a clinical setting. Multiple purifications of intermediates are commonly required, and the final step, involving linkage to protein lysine residues, usually results in 30-60% yields, necessitating a further purification step prior to patient administration. In addition, these methods result in fluorinated targeting species which accumulate in the kidney, somewhat like radiometals.
It was recently reported that .sup.18 F-fluoroiodomethane (.sup.18 FCH.sub.2 I) is a useful intermediate for the fluorination of organic intermediates. Zheng et al., J. Nucl. Med., 38: 177P (Abs. 761) (1997). In this process, diiodomethane is fluorinated with the F-18 ion by a room temperature reaction in acetonitrile solvent, resulting in up to a 40% yield. The .sup.18 FCH.sub.2 I is then distilled into reaction vials containing various strong nucleophiles in anhydrous acetonitrile and allowed to react at 80.degree. C. for fifteen minutes. Nucleophilic attack by carboxylates, thiolates, phenolates, and amines in particular, replaces the remaining iodine of .sup.18 FCH.sub.2 I, with overall yields of 10 to 35%. The reaction products can be purified by reverse-phase HPLC. Fluoromethyl diethylamine, fluoromethyl benzoate, fluoromethyl benzyl thioether and fluoromethyl 4-(2-hydroxy-3-aminopropoxy)-carbazole have been made by this method.
As discussed above, the currently available methods for labelling protein-based targeting vectors with F-18 are unsuitable. There is a need, therefore, for a simple, efficient method for incorporating the F-18 radionuclide into peptide-containing targeting vectors, such as proteins, antibodies, antibody fragments, and receptor-targeted peptides, to allow the use of such targeting vectors in routine clinical positron emission tomography.